The present invention relates to protective coatings for surfaces of silver or silver alloys and especially to protected silvered substrates such as front surface mirrors and low emissivity glass utilizing the same.
As is known, e.g. from U.S. Pat. No. 3,687,713 it has been the general practice to fabricate mirrors by applying reflective metals such as silver, platinum, gold, copper, aluminum, chromium, rhodium and the like, by vacuum deposition, onto siliceous substrates such as glass and for some uses onto metal substrates and plastic substrates. Over the years it has been observed that these reflective metals do not adhere satisfactorily to siliceous substrates and that such reflective metal surfaces frequently develop pin holes or blisters and/or peel away in large areas.
Attempts have been made in the prior art to correct this problem of poor adherence by employing an overlay of protective lacquer or shellac. These attempts, for the most part, have not been satisfactory since the shellac or lacquer has not been impervious to moisture, or changes in humidity and temperature, and because overcoatings do not improve the adherence between the mirror coating and the substrate.
When such a mirror has been subjected to repeated changes of humidity and/or temperature, the reflective metal has usually blistered and peeled off. In addition a shellacked or lacquered mirror is limited in its use to a back-view mirror, most of which have been made by chemical reduction processes which are not suitable for front-view mirrors in any case.
In another attempt to correct poor adherence, a thin layer of metallic lead oxide has been bonded to a substrate and the reflective metal applied thereto. However, the metallic lead oxide mirror will not remain intact if it is subjected to cycles of humidity and temperature changes. In addition, in a back-view mirror the thickness of the metallic lead oxide necessary to provide a good adherence is sufficiently deep that an undesirable amount of incident light, passing through the metallic lead oxide layer, is attenuated. Further, the metallic lead oxide layer does not provide any advantage if it should be used as a front surface mirror.
It is known that initially the reflectivity of a vacuum deposited film of silver is greater than 95% through most of the visible spectrum as well as through a good portion of the infrared light spectrum. Despite this high reflectivity, unprotected silver has not been used successfully as a front view mirror because in addition to its poor adherence to siliceous substrates it is soft (and therefore scratches) and tarnishes easily. Thus, unprotected silver films will not stand handling and cleaning and the effects of the atmosphere, particularly the sulfides contained in the atmosphere which quickly tarnish the silver.
Especially in the solar energy field, it is often desirable to reach a high concentration of solar radiation onto a solar receiver. This is often done by means of reflectors which must be highly specular with good solar reflectivity, yet have a relatively low cost. Second surface glass mirrors using regular glass have the disadvantage of reflection losses due to the absorption of the solar radiation into the glass superstrate. Thus up to 12% of solar radiation is absorbed into 4 mm thick glass. Expensive overcoatings on the non-reflective surface of the mirror are needed in order to achieve an expected lifetime of 30 years under outdoor conditions. Regular glass can be replaced by very expensive low-iron glass, but this will increase the cost of the reflector radically, even though the reflectance will typically be improved by 7-10% in a 4 mm thick glass superstrate. Again, the expensive lacquer remains on the non-reflective surface to protect the metal film.
The use of highly reflective front surface mirrors eliminates the need for expensive low iron glass to reach high reflectivity and expensive lacquer for back protection. Maximum reflectivity is typically improved by 2%. Alternatively, a plastic substrate can be employed when using front surface mirrors, as the problem of UV stabilization and transparency does not have to be dealt with. There is an additional advantage in that currently many mirrors use glued mounting elements to attach the reflectors to the mechanical structure. This mounting element is glued onto the protective coating of the mirror, which causes adhesion problems. With front surface reflectors, the mounting element can be directly connected to the glass substrate, or alternatively, can be a built-in element of an injected plastic substrate, which is far simpler and very much more secure.
Many previous works exist on Front Surface Mirrors. As high solar reflectivity is required, only silver and aluminium reach a solar reflectivity high enough to be good candidates as reflecting surfaces. Solar reflectance of unprotected freshly evaporated aluminium is less than 92%, and of freshly evaporated silver almost 98% (AM 1). At AM 1.5 solar spectrum, this last value will be even higher. Therefore silver is chosen as the reflecting surface. However, silver tarnishes quite easily due to atmospheric effects and therefore needs to be protected from mechanical and chemical attacks. Mechanical and optical properties of dielectric films have been investigated in the past and alumina (Al.sub.2 O.sub.3) and quartz (SiO.sub.2) films have shown very high levels of hardness and transparency in the solar spectrum, as described in Reflectance and Preparation of Front Surface Mirrors for Use at Various Angles of Incidence from the Ultraviolet to the Far Infrared. G. Hass. Jour. Opt. Sc. Am. Vol. 72 (January 1982)
Different ways of applying this kind of material have been studied, in particular evaporation, sputtering and electron beam techniques as described in the above article and by J. T. Cox, H. Hass, J. B. Rawsey. Improved Dielectric Films for Multilayer Coatings and Mirror Protection, Journal de Physique. Vol. 25 (January-February 1964).
In U.S. Pat. No. 3,687,713 there is described and claimed a protective coating means on a substrate, comprising in combination, a substrate having an outer surface of substantially silver, a layer of aluminum oxide secured to said outer surface, and a layer of silicon dioxide secured to said layer of aluminium oxide.
According to said patent the protective coatings of Al.sub.2 O.sub.3 and SiO.sub.2 result in good durability to abrasion and insensitivity to moisture and other environmental conditions, which normally affect such devices.
The design used in said patent is based on several properties of different materials. First the silver is secured to the substrate. If using glass or SiO.sub.2 substrate, a first layer of chromium and nickel alloy or alumina is recommended for use. After applying the silver, a protective layer of alumina is deposited. This layer is very hard but is sensitive to moisture because of its high porosity. The multilayer is then coated with a thin layer of quartz that "seals" the mirror and protects it from moisture attack. The thickness of the different layers must be adapted according to the trade-off between mechanical (thick) and optical (thin, interference effects) performance.
Having prepared several mirrors of this kind it was found, as described also in comparative example 3 hereinafter, that the coating peels off under outdoor use especially as a result of failure of this alumina-silver bond.